Flat panel displays such as liquid crystal displays (LCDs) or organic light emitting diode (OLED) displays are useful in a wide variety of applications under a wide variety of environmental conditions. When viewed in a dark environment (little ambient radiation), such displays need not be as bright as when viewed in a lighter environment (more ambient radiation). If the display light output is adjusted periodically to compensate for ambient light conditions, the display can maintain a fixed ratio between the ambient and displayed light even if the ambient light changes. This can, in turn, increase display brightness to improve visibility in a bright environment and increase display device lifetime and reduce power usage by reducing unnecessary display brightness in a dark environment.
The use of photosensors with displays to detect ambient light and adjusting the brightness of the display in response to ambient illumination is known. Efficient silicon photosensors are available and generally provide a current proportional to the light incident on the sensor. These photosensors are constructed on silicon substrates. Such sensors can be combined with displays to provide ambient sensing. For example, see JP2002-297096-A, which describes a circuit for providing ambient compensation to an electroluminescent display. However, as implemented, the sensor is separate from the display and senses the light at a single point. This increases the cost, number of components, and size of the device; reduces the sensitivity of the sensor; and does not directly measure the light incident on the display itself.
It is known to integrate a light sensor on an active-matrix display device for the purpose of sensing light emitted from the display device itself. See for example, U.S. Pat. No. 6,489,631 issued Dec. 3, 2002 to Young et al., which describes a display having integrated photosensors for sensing light emitted by a light emitting element of the display. However, the arrangement of the sensor coupled with a light emitter limits the size of the photosensor and its ability to sense ambient light. Moreover, such photosensors constructed on flat panel displays do not have the efficiency of those constructed on silicon substrates and do not have the sensitivity necessary to provide a signal representative of lower light levels, for example <100 cd/m2, where displays are often used. Hence, alternative circuits and designs are necessary.
When providing ambient compensation to a display, it is important that the light sensing device provide a continuously valid output that is always representative of the ambient illumination. If, instead, the output is periodically invalid, any compensation will be periodically incorrect and may cause flicker in the display. Alternatively, additional circuitry must be added to sample and hold the output of the light sensing device. Moreover, it is advantageous to provide a signal output that is representative of the ambient illumination over a range of light levels.
There is a need therefore for an improved photosensor for the detection of ambient light within an active matrix flat panel display.